A digital micromirror device (DMD) is a type of microelectromechanical system (MEMS) device. More specifically, the DMD is a fast response, reflective digital spatial light modulator. It can be combined with image processing, memory, a light source, and optics to form a digital light processing system capable of projecting large, bright, high-contrast, color images.
A DMD may be fabricated using complementary metal-oxide semiconductor (CMOS)-like processes over a CMOS memory. A typical DMD comprises an array of individually addressable mirror elements, each having an aluminum mirror that can reflect light in one of two directions, depending on the state of an underlying memory cell. These mirrors may be fabricated over address circuitry that has static random access memory (SRAM) cells and address electrodes. Each mirror forms one pixel and is bistable, such that light directed upon the mirror will be reflected in one of two directions. In an “on” mirror position, light is reflected to a projector lens and focused on a display plane. In the “off” position, light is deflected to a light absorber. The array of “on” and “off” pixels produces an image. Color is achieved by using color light sources or filters, either stationary or rotating, in combination with one or a plurality of DMD chips.
Generally, with the SRAM cell in a first state, “on,” the mirror rotates to +12 degrees, and with the memory cell in a second state, “off,” the mirror rotates to −12 degrees. Reset drivers are used to transition the mirror to the appropriate state of the memory cell.
Generally, a bipolar reset waveform is generated by the reset drivers. Integration of the reset drivers generally requires active devices in order to switch the higher voltage levels in the bipolar reset waveform. One disadvantage of the prior art is that the integration of the bipolar reset drivers and associated circuitry onto the DMD CMOS substrate may not be accomplished in a normal CMOS process due to the large magnitude of a negative reset voltage in the bipolar waveform. In particular, the reset voltage, which is negative with respect to the substrate, generally will forward bias PN junctions with respect to the substrate. Another disadvantage is the cost of processing that is involved in integrating the bipolar structures on the CMOS substrate.